1. Technical Field
The principles of the present invention relate to banding artifacts which can be present in digital video.
2. Description of Related Art
Digital video is typically captured or produced at high bit depth (16 bits per component for animated content). However, current video encoders and display devices have been designed to compress signals with 8 bits per component or less. The bit depth conversion from higher to lower bit depth can easily create banding artifacts. That is, banding artifacts are perceived as bands of distinctive colors in what should be a gradual color transition.
Similarly, banding artifacts can also be created by the process of compression. Video compression methods aim at finding efficient coding representations for digital video. This can be done by removing the inter-redundancies in both spatial and temporal domains of video contents. Because the human visual system is less sensitive to high frequency components of video, information of the high frequency components is reduced by the quantization process. Even though removing redundancy provides a high compression ratio, the visual video quality is degraded with the introduction of artifacts at the decompressed picture. Banding artifacts can also be introduced in this process. Banding artifacts occur when the quantization process suppresses the film grain or distorts the original contents, which then form the contour-like artifact. This often appears in animation contents and film contents with smooth film grain and smooth gradient changes.
In non real-time encoding applications such as Digital Versatile Disk (DVD) authoring, visual quality is the most important target. Furthermore, with the introduction of high definition formats and higher resolution displays, banding artifacts become more visible to the audience. In order to alleviate or remove banding artifacts, the locations of the banding artifacts must be spotted and evaluated. Using human effort to perform the detection task is not desirable because it consumes a lot of time and energy. Moreover, human detection of banding artifacts is prone to human errors resulting in missing existing artifacts.
As a consequence, there is a major need for methods and the like, which can automatically detect and evaluate the existence of banding artifacts in the reconstructed video contents. Such inventive concepts can dramatically reduce time spent in evaluating the artifacts and enhance the quality of the compressed video.
There are not many solutions that consider the detection and correction of banding artifacts in video contents. In one known method, two-stage contour detection with re-quantization and directional contrast features is proposed. Such a method first removes smooth regions and then applies feature-based filtering to separate false contours. However, this method does not take the original video contents into account for determining banding artifacts. Therefore, such a method can falsely declare banding effect intended by the video producers as banding artifacts. Then, the application of filters to remove these intended banding negatively affects (or degrades) the quality of the encoded video.
As a consequence, there is a need for a new and effective method for detecting banding artifacts that overcomes the shortfalls of the known methods.